1. Field of the Invention
The present invention relates to a communication system using a passive optical network (PON), and more particularly to a communication system using a wavelength division multiplexed passive optical network (WDM PON).
2. Description of the Related Art
A number of architectures and methods have been proposed to configure a subscriber network from a central office to buildings or general homes, such as x-Digital Subscriber Line (xDSL), Hybrid Fiber Coax (HFC), Fiber To The Building (FTTB), Fiber To The Curb (FTTC), and Fiber To The Home (FTTH), and the like.
The above FTTB, FTTC and FTTH, hereinafter collectively “FTTx,” can be classified into two categories. First, an active FTTx configured by an active optical network (AON). Second, a passive FTTx configured by a passive optical network (PON). The PON is proposed as a cost-effective optical network for the near future. The PON is based on a communication network architecture having a point-to-multipoint topology based on passive devices. Thus, the PON has the flexibility meet increased communication demand and provide stable communication services to a plurality of subscribers. This is accomplished with the development and supply of various communication media and high-definition and large-capacity digital media technologies such as digital satellite broadcast, and the like.
The PON has a subscriber network architecture that forms a tree-type distribution topology. It couples one optical line terminal (OLT) to a plurality of optical network units (ONUs) through a 1×N passive optical splitter. Such a PON includes a remote node (RN) for dividing a downstream optical signal received from a central office (CO), acting as a conduit for a service provider, into multiple signals. The PON also transmits the multiple signals to the respective subscribers. Moreover, the CO is coupled to the RN over a single optical fiber. The RN is coupled to the subscribers over independent optical channels. PON systems may be based on time division multiplexing (TDM) or wavelength division multiplexing (WDM).
TDM divides the bandwidth of a link into separate channels or time slots. In TDM, subscribers must carry out a synchronization operation in the CO, according to a time criterion. More particularly, when communication demand increases, the TDM mechanism cannot easily meet the increased communication demand due to technical and economical factors. In contrast, WDM enables the CO to simultaneously transmit data items having different wavelengths to the subscribers. In WDM, each of the subscribers can carry out two-way communications using an assigned wavelength signal.
FIG. 1 is a block diagram illustrating a conventional wavelength division multiplexed passive optical network (WDM PON) system. Referring to FIG. 1, the conventional WDM PON system includes a central office (CO) 110, an optical fiber 140, a remote node (RN) 120 and a plurality of subscribers 130.
The CO 110 includes a′ plurality of optical transmitters 111 for outputting downstream optical signals that are transmitted to the subscribers 130, a multiplexer/demultiplexer (MUX/DEMUX) 113, and a plurality of optical receivers 112. In a conventional WDM PON system each of the subscribers 130 performs two-way communications using a specific assigned wavelength. Thus, confidential communication security is ensured, and if desired, the communication network can be easily and promptly extended.
As shown in FIG. 1, the optical transmitters 111 output downstream optical signals having a range of wavelengths λ1˜λn to the MUX/DEMUX 113. The MUX/DEMUX 113 multiplexes the downstream optical signals and then outputs the multiplexed downstream optical signals to the RN 120.
Light sources for the optical transmitters 111 include: (1) coherent laser light sources such as a distributed feedback laser array, (2) Multi-Frequency Laser (MFL), (3) spectrum-sliced light source, (4) mode-locked Fabry-Perot laser with incoherent light, and the like.
The distributed feedback laser array, MFL, etc. has a comprehensive range of selectable wavelengths. However, the distributed feedback laser array must include a separate device for ensuring wavelength selectivity and stability. In contrast, the Fabry-Perot laser light source, etc. has a limited range of selectable wavelengths. However, it can easily produce coherent light, but cannot provide multiple channels. Furthermore, the Fabry-Perot laser light source is seriously degraded in performance due to a mode partition noise. This results from the Fabry-Perot laser light source modulating a spectrum division signal and then transmitting the modulated signal at a high speed.
Incoherent light sources such as a light emitting diode (LED), a super-luminescent LED, etc. other than the above-described coherent light sources have a comprehensive range of available wavelengths and are inexpensive. However, since the modulation bandwidth for the incoherent light sources is narrow and output power for the incoherent light sources is low, incoherent light sources are inadequate for performing long-distance communication.
To address these problems, a method for employing an optical fiber amplifier for generating amplified spontaneous emission (ASE) light as a light source, etc has been proposed. However, when the ASE light source is used, a separate external modulator such as LiNbO3, etc. must also be provided.
The optical receivers 112 are configured by photodiodes, etc. They convert upstream signals received through the MUX/DEMUX 113 into electric signals.
The MUX/DEMUX 113 can be configured using a wavelength division multiplexer such as an arrayed waveguide grating (AWG), etc. The MUX/DEMUX 113 includes a first port located at one end coupled to the optical fiber 140, and a plurality of second ports located at the other end. The MUX/DEMUX 113 outputs an upstream optical signal from the first port to the second ports. It also outputs downstream optical signals from the second ports to the first port. However, since such an AWQ etc. changes or shifts wavelengths in response temperature, there must be a separate temperature control device for controlling the AWG's temperature. Consequently, the size of the system and its manufacturing cost are increased.
The RN 120 is located between the CO 110 and the each subscriber 130. It demultiplexes a downstream optical signal received from the CO 110 into multiple downstream channels. Thereafter, it transmits the multiple downstream channels to the subscribers 130. Furthermore, the RN 120 multiplexes upstream optical signals received from the subscribers 130 and transmits the multiplexed upstream optical signals to the CO 110. The RN 120 is configured by a wavelength division demultiplexer/multiplexer (DEMUX/MUX) such as an AWC; and the like.
Each subscriber 130 includes an optical transmitter 132 and an optical receiver 131 for receiving a corresponding downstream optical signal from the RN 120.
Optical receivers 131 can be configured by photodiodes. The optical receivers 131 convert downstream optical signals having wavelengths λ1˜λn received through the RN 120 into electric signals.
Optical transmitters 132 use coherent light sources of laser diodes, etc. They use a range of wavelengths different from the range of wavelengths of the downstream optical signals. In particular, the upstream optical signals are outputted from the laser diodes having a range of wavelengths λn+1˜λ2n.
In conventional WDM PON systems, a CO assigns different wavelengths to respective subscribers. In this manner communication security is ensured and the performance of such a WDM PON system can be enhanced. Furthermore, since the WDM PON system uses laser diodes, the optical-energy density is high, long-distance communication can be performed, and wavelength selectivity can be enhanced. However, the WDM PON system must be equipped with a separate external modulator or a separate device for enhancing wavelength stability and selectivity. Thus, the size of the WDM PON system is increased and the WDM PON system cannot be implemented cost-effectively. Further, since such a WDM PON system must be equipped with separate devices to ensure selectivity between wavelengths to perform long-distance communication and ensure multiple channels, subscribers must share expenses.